What Are Floor Joists In Construction?

Floor joists are important to learning the basics of building a house which can be extremely beneficial to you. Although it’s not necessary to know any of it, you can feel more at ease knowing how everything is done to create your home.

When it comes to floor joists, there isn’t a lot the average person needs to know. However, it’s always nice to know the basics because this just might be the most important part of keeping your home from collapsing. 

What Are Floor Joists?

What Is A Floor Joist?View in gallery

A floor joist is a horizontal beam, usually made of wood, that is used in framing to support and enclose the flooring above the foundation. Their job is to transfer loads to the vertical beams of the house.

Most of the floor joists will either be single or doubled up, and most often made of 2x6s or 2x8s. Though on occasions, they are made of 2x10s and tripled up for extra support. But they are never less than 2×6 joists.

Using floor joists isn’t always necessary for a house however because many floors are built directly onto a concrete slab. But houses built over basements or onto wood foundations need floor joists for structural integrity. 

Floor Joists Spacing

Floor Joist SpacingView in gallery

This is the most important part of installing floor joists. The spacing means everything as if they are spaced too far apart, they are practically useless, but if they are spaced too close, you will overspend on joists. 

There are two things that affect the spacing of floor joists. The first thing is lumber grade, which can change the outcome by an inch or two. But what changes it drastically is the second thing, which is board size.

While the spacing codes have changed over the years, this is where we stand today. Here are the three most common spacings when it comes to floor joists and covers 90% of all types of spacings for floor joists. 

  • 2×6 – 12 inches
  • 2×8 – 16 inches
  • 2×10 – 24 inches

You’ll notice that we stop at 2x10s. This is because joists should never be spaced more than 24-inches apart. So both 2x12s and 2x14s need to be spaced no further apart than 24-inches just like the 2x10s. 

Some agree that shorter widths should be further apart too. But this is a general estimate depending on grade and selected structural integrity. The vertical boards and lengths of the horizontal boards matter too. This is a simple guideline. 

Are Floor Joists Load-Bearing?

Yes, the floor joists’ job is to be load-bearing. They act as the support for the upper levels, often transferring weight to the vertical supports of the foundation. But when it comes to upper levels, the floor joists support the walls. 

Then, the walls support the next floor. However, if a wall is placed parallel to the joists, it isn’t load-bearing. Only walls placed perpendicular to the floor joists can be load-bearing. For more info on load-bearing walls, check this article out

But what you really need to know is that floor joists bear the weight of the entire home. While walls usually only bear part of the weight, with some not bearing any weight at all. Floor joists are important for this reason alone. 

Types Of Floor Joists

Types Of Floor JoistsView in gallery

There are a few types of joists used in modern-day floor joists. While there are some outdated floor joist options, there are typically only three types used by contractors in wooden structures. This is what you’ll see most often. 

Solid Lumber

Solid lumber is probably the cheapest type of floor joist if you use standard wood, rather than specialty wood. But in general, solid lumber is more fire-resistant and water-resistant than most other options.

Because of its price and a large array of options, solid lumber is the most popular type of floor joist. It’s easy to use, understand, and is the simplest type of floor joist. If you know anything about construction, then you can’t mess it up. 


I-joists look like the letter I. The top and bottoms of the I are made with either solid wood or laminated veneer, but the center is made with plywood or OSB. They can span a longer distance than solid lumber.

Because they are made with plywood, they can be cheaper than other options and can be stacked and glued to create a stronger board than what one single layer of plywood might be. This makes I-joists very strong. 

Open-Web Floor Trusses

Open-web floor trusses are made with 2x4s on the top and bottom with a “web” of diagonal boards in the middle. The boards are secured with metal plates. Because of their design, they use less wood than normal.

They also have gaps to leave plenty of room to make wiring and plumbing much easier. However, crawlspaces are tricker with web floors because there isn’t usually enough room to crawl through like there would be with solid wood. 

Building Codes For Floor Joists

Local building code authority usually requires your joists to support at least 10 pounds per square foot for a dead load and 30 pounds per square foot for a live load. Different woods, grades, spans, and so on affect the support per square foot. 

At this point, you’re probably wondering what the difference between a live load and a dead load. After all, how can there be two different load guidelines? Do you only focus on the heavier one? Sounds complicated, but the answer is quite simple. 

A dead load is a permanent load that is always there as it comes from the structure and load from the building. But then live loads are temporary loads. They are applied over time, including traffic, furniture, and more. 

Repairing Floor Joists

In general, floor joist repairs should be done by a professional. This is the structural integrity of your entire home that is being compromised and needs to be done by someone with experience with floor joists. 

For simple repairs to floor joists, consider using a flitch plate. A flitch plate is a thin piece of steel or plywood that is bolted to a weak joist to offer support. However, another option is to use an identical board and secure it to the old one.

You can still leave the old board there as the new board is just there to hold it in place and to become the new load-bearing board in the area. Nothing is compromised and you don’t have to take any risks.

Picking The Best Boards For Floor Joists

Picking The Best Boards For Floor JoistsView in gallery

Choosing boards for floor joists can be overwhelming considering how important these joists will be. But there are only a few things to consider when choosing them, and they are all fairly simple decisions. 

It’s important to talk to a professional if picking boards out for the first time. They can walk you through the steps and go over each of these factors to ensure you are choosing the best boards for the job and your budget. 

Type Of Wood

Different wood species have different characteristics. Some have more elasticity while others have more raw strength. Most of the time, slow-growing trees are stronger and are hardwood trees, while fast-growing trees are softwood trees.

Stronger trees that still have good elasticity are ideal for floor joists. Some popular choices include pine, fir, and spruce. These hardwoods, which you’ll notice are conifers, are great options and are easy to find.

Grade Of Wood

The grade of wood can be very important because the lower the grade (the higher numbers) have more defects and aren’t structurally strong. So the lower the number, the better the wood will be. This is not a time to fudge on the rules.

That said, using Grade-1 isn’t necessary as Grade-2 is flawless enough for almost any project, including floor joists. But try to avoid using anything that is Grade-3 or lower as it will risk the integrity of your foundation. 

Size Of Wood

When it comes to the size of the boards, avoid using anything smaller than a 2×6. The largest you will use is a 2×12, but most of the time, the boards will either be 2×8 or 2×10. Though 2x6s are common, they don’t always save you money.

Because when you choose 2x6s, you have to space them closer together, which would be less than 16″ while you can space 2×12 24″ apart sometimes. So doing the math on spacing is very important to figure costs. 

Can I Put In my Own Floor Joists?

It’s probably best that you leave floor joist installation to the professionals. You can paint your own walls and even put in your own non-load-bearing walls. But when it comes to structural integrity, it’s always worth hiring a contractor. 

If you like, you can work with the contractor if they allow it. This way you can learn what you want to know while still having the security of someone there who was trained to install floor joists in residential homes. 

The same goes for electricity and water. You wouldn’t put in your own septic. So keep a set list of things that you hire a contractor for. In the end, it will save you money, time, and a whole lot of restless nights.